The definition of molecular sieve, according to Szostak, "Molecular Sieves, Principles Of Synthesis And Identification," 1989, Van Nostrand Reinhold, New York, N.Y., at page 3, is --
"A molecular sieve framework is based on an extensive three-dimensional network of oxygen ions containing generally tetrahedral-type sites. In addition to the Si.sup.+4 and Al.sup.+3 that compositionally define the zeolite molecular sieves, other cations also can occupy these sites. These cations need not be isoelectronic with Si.sup.+4 and Al.sup.+3, but must have the ability to occupy framework sites. Cations presently known to occupy these sites within molecular sieve structures are --
The term molecular sieve encompasses the variety of structures within the classification set forth in FIG. 1.1 of Szostak, supra, page 2, which classification is incorporated by reference. Molecular sieves come in two varieties, zeolitic molecular sieves ("ZMS") and non-zeolitic molecular sieves ("NZMS"). Szostak (page 4) treats aluminosilicates generally to be ZMS provided there is at least one aluminum ion per unit cell based on the bulk composition of the sample. The remaining structures are recognized to be NZMS. According to this characterization, the ZSM-5 structure is considered a ZMS at silica/alumina less than 190 and NZMS above 190. This same convention holds when ZMS's contain trace amounts of other elements in the framework ion positions. (See Szostak, page 5, who considers a crystalline structure a ZMS if the number of other cations in the framework, other than aluminum and silicon, averages less than one per unit cell, all others being a NZMS.)
Zeolitic molecular sieves are typically made from a source of silica that is reacted with a source of aluminum, in the presence of materials that insure significantly alkaline conditions, water and .sup.-- OH. The mix of the reactants may be called the reaction's nutrients. Many of the reactions are conducted in the presence of an organic template or crystal-directing agent, which induces a specific zeolite structure that can not be formed in the absence of the organic template. Most of the organic templates are bases, and many introduce hydroxyl ions to the reaction system. The reaction involves a liquid gel phase ("soup") in which rearrangements and transitions occurs, such that a redistribution occurs between the alumina and silica nutrients, and structural molecular identities corresponding to specific zeolites or other molecular sieves are formed. It is known that zeolites are not often formed above 350.degree. C., though descriptions of higher temperature formation of certain molecular sieves has been mentioned in the literature (see Szostak, supra, page 52). Lower temperatures than about 100.degree. C. require extensive crystallization time. As Szostak, supra, page 54, points out --
When the desired crystal structure is obtained, the molecular sieve is brought to ambient temperatures and the crystallization process is arrested. The product of the reaction is isolated typically as a loose powder. The crystals that are formed in the powder are so assembled in the structure as to form special micro pores and micro pore openings of a kind that distinguishes the structure. The resulting crystals are an assemblage of individual units the growth of which may be small, medium or large, depending on the conditions employed in the traditional method. The crystals may then, in the usual case, be formed into composite structures that allow their use as, e.g., absorbents and catalysts.
A number of references describe processes for making ZMS by reacting an amorphous precursor in the presence of a small amount of water to form a dense interbonded mass. The amount of water is selected to be less than that which is used in the aforementioned traditional method but large enough to interbond the ZMS particles into dense masses.
For example, Haden et al., U.S. Pat. No. 3,065,064, convert to a ZMS, a dehydrated kaolin clay having a SiO.sub.2 /Al.sub.2 O.sub.3 mol ratio of about 2, in the presence of a "concentrated aqueous solution of NaOH." The H.sub.2 O to Na.sub.2 O mol ratio in the mixture is within the range of 4.5-11.5 "and being present in an amount such that the Na.sub.2 O/SiO.sub.2 mol ratio in the mixture is about 0.5. " According to Haden et al.:
According to the patentee, the object of the process is to form the ZMS into a "compact mass or masses" which is defined as a "dense or substantially nonporous mass." The patentee states,
Miller, U.S. Pat. No. 5,558,851, patented Sep. 24, 1996, relates to shaped zeolite structures wherein the reactants, in making the structures, are formed into a water-wet thick paste and crystallized after forming into a shaped structure. According to Miller's process, and as practiced by Haden et al., crystallization occurs without the presence of an "external liquid phase.".sup.1 Needless to say, Miller's process transforms the reactants into the typical hydrothennally induced liquid phase reaction. According to Miller, the zeolites in the shaped structures contain "very small crystallites." Miller's use of low amounts of alkali metal should be expected to generate smaller crystallites (See Szostak, supra, page 73). As is the case with Haden et al., supra, Miller interbonds the ZMS particles so as to form the dense mass structure described by Haden et al. FNT .sup.1 Compare with the "in the absence of an aqueous liquid phase external to and in contact with the mass" description of Haden et al., supra.
According to Miller's example 16 (specifically at column 21, lines 1-20), a mixture of TPAOH, NaOH and water were combined with a mixture of silica and sodium aluminate, and the combination was mixed for 3 hours. A paste was formed of the combination by the addition of more water. Miller then combined the technique of Haden et al., supra, by the addition of kaolin clay, and he reduced the volatiles level to 53 weight percent by continued mixing at 60.degree. C. Miller then air-dried the mixture to form a powder of 48 weight percent volatiles. According to Miller, "[t]he molar ratio of H.sub.2 O/SiO.sub.2 at this point was about 2.5. " It is assumed that the powder was an aggregation of pasted particles that no longer exhibit the particulateness and porosity of the silica. Miller placed the powder into a Teflon bottle and the bottle was placed "in a stainless steel pressure vessel and heated at 140.degree. C. and autogenous pressure for two days." According to Miller, the "resulting product was washed with water, dried overnight in a vacuum oven at 120.degree. C., and calcined in air for three hours at 593.degree. C." Miller states that "X-ray diffraction analysis showed the product to be nearly 100% ZSM-5. The average crystallite size by SEM was about 0.1 micron."
Ramesh B. Borade and Abraham Clearfield, ("Borade and Clearfield"),.sup.2 make a zeolite Beta in a 24 hour synthesis at 170.degree. C. from an extremely dense system in which the weight ratio of solid, measured as sodium aluminate and silica, to liquid, measured as tetraethylammoniumn hydroxide and water, mixtures is 1:1.8. They found that the product has comparable catalytic properties to samples prepared by previous methods. According to the detail description of Borade and Clearfield, the amount of the aforementioned liquid component "is just sufficient to wet all the solid particles and in some cases (especially at Si/Al ratio&lt;10) the reaction mixture is in the form of small lumps." According to the authors, the process uses "a much smaller proportion of TEAOH and shorter reaction time as compared to the usually synthetic methods." Lowering the proportion of TEAOH allows the decrease in the amount of water content. "This change increases the weight ratio of solid to liquid in the systems from 1:9.1 to 1:1.8. The mole ratios of SiO.sub.2 /TEAOH increased from 0.53 to 6 and H.sub.2 O/SiO.sub.2 decreased from 23 to 6.1. " FNT .sup.2 "Synthesis of zeolite Beta from dense system containing a minimum of template," Catalysis Letters, 26 (1994) 285-289.
The procedure employed by Borade and Clearfield is as follows: "Sodium aluminate, TEAOCH and water were mixed and stirred for about 15 minutes. Then, this solution was added to a highly reactive fumed silica and stirred with a spatula for about 15 minutes. Initially, the mixture appears to be a dry powder. As stirring continued for about 2 hours the mixture turned into a very dense and thick solid (solid: liquid .sup..about. 1:2), which was transferred into a stainless-steel autoclave and heated at 443K and autogenous pressure for 24 hours. The pH of the initial reaction mixture was in the range 13.2-13.8 and after crystallization it was in the range 11.4-12.0. After synthesis samples were dried at 120.degree. C. and then calcined at 540.degree. C. for about 15 hours." According to the authors, "[T]he marked reduction in the use of TEAOH in the present method (2.5 versus 10-28 moles of TEAOH with reference to 1 mole of Al.sup.2 O.sub.3), shorter crystallization time (24 hours versus 4-10 days) and increased productivity (per batch) should lead to a lower synthesis cost of zeolite Beta. The product obtained also has comparable catalytic properties at least with reference to the cracking of hexane."
The methods of Haden et al., U.S. Pat. No. 3,065,064, Miller, U.S. Pat. No.5,558,851, and Borade and Clearfield, take a common approach to making ZMS, by reducing the amount of gelation of SiO.sub.2. This is accomplished by the use of smaller amounts of water than has been used in the traditional process for making ZMS. As a result, Borade and Clearfield produce a very dense and thick solid, Miller produces a paste that forms a dense and thick solid, and Haden et al. produce a dense or substantially nonporous mass.
In effect, these authors have developed a process that depends on surface gelation of the silica particles for ZMS formation. Surface gelation occurs at the particle interior and exterior surfaces. It allows the particles to become glued to each other by interfacial wetting of one viscid exterior surface by another. It also affects the interior pores by filling or collapsing pores thereby reducing the particle's pore volume.
Surface gelation allows Haden et al. and Miller to make shaped ZMS structures. However, such surface gelation as pointed out by Haden et al. results in a relatively nonporous structure, meaning that the macro and meso pores.sup.3 of the silica are eliminated by the gelation and bonding that occur in their processes. FNT .sup.3 As used herein, "macro pores" means above 500 angstroms (over 50 nanometers); "meso pores" means 20 to 500 angstroms (2-50 nanometers); and "micro pores" means less than 20 angstroms (under 2 nanometers). Also, in the terms describing the invention of this application, the terms "large pore" and "large pores," encompass structures that contain macro and/or meso pores.
It is well known that molecular sieves are made from cation-oxide containing materials, such as silica and alumina, which prior to hydrothermal conversion contain macro and meso pores and a geometric framework that surrounds and formns such pores. In the thermal conversion of these porous materials, the framework is degraded and the framework becomes part of a viscid (gelatinous) soup, no longer capable of providing the macro and meso pore network. The typical product of the typical hydrothermal conversion is a powdery precipitate.
Haden et al., U.S. Pat. No. 3,065,064, Miller, U.S. Pat. No. 5,558,851, and Borade and Clearfield, operate a "non-soup" hydrothermal process designed to create an amount of surface gelation that essentially eliminates the porosity inherent in the precursor cation-oxide containing framework materials in order to form a paste and achieve increased density.
There is described in the heterogeneous catalyst art a number of processes for forming a catalyst. One method is characterized as an impregnation of the catalyst support with liquid reagents that deposit in the pores of the support. It is commonly known as an impregnation process. The amount of liquid reagent deposited on the support leaves an apparent incipient wet film. Those ingredients invariably react on the surface of the support to form the catalyst. The support is not changed by virtue of such treatment, and retains its original porosity. In most cases, the support is not thought to react with the liquid reagents. (See Stiles and Koch, Catalyst Manufacture, Second Edition, 1995, published by Marcel Dekker, Inc., New York, N.Y. 10016.)
There is no process known in the art that causes conversion of conventional molecular sieve forming nutrients that retains macro and meso porosity characteristics of a geometric framework cation-oxide containing nutrient. It would be desirable to produce molecular sieves that possess a framework morphology characteristic of at least one of cation oxide containing nutrients in the reaction. It would be desirable to have a reaction of the typical nutrients in the formation of a molecular sieve that retains the framework morphology characteristic of at least one cation oxide containing nutrients in the reaction.